Gas scrubbers are devices used for separating components of a gas admixture. In some embodiments, these devices are used to “purify” gasses or, stated in the alternative, remove undesirable components from a gas stream. For example, primitive scrubbers have been used since the inception of submarine warfare to remove carbon dioxide from the air supply in the submarine.
More recently, gas scrubbers have proven to be essential in many industries. For example, gas scrubbers are used to prevent pollution from the burning of coal during power generation. Gas scrubbers are also used to remove undesired components from crude oil during refining and to remove undesirable components from process gas streams during the production of chemicals, metals and devices such as semiconductors and the like. More specifically, caustic scrubbers use aqueous caustic, i.e. sodium hydroxide (NaOH) and/or potassium hydroxide (KOH) to “scrub” or react with hydrogen sulfide (H2S) to remove it to form NaHS (aqueous) and water (liquid), which consumes the caustic.
Carbonyl sulfide (COS) is produced by furnaces in petroleum and/or petrochemical processes. The presence of COS will not only cause fouling in the caustic system, but will also cause an undesirable side effect on the downstream side of the caustic tower. The side effect may or may not be limited to poisoning the hydrogenation catalyst, thus increasing the sulfur number of subsequently produced pyrolysis gasoline (pygas), a naphtha-range product with a high aromatics content. The current method to remove COS is to use an absorbent, including, but not necessarily limited to molecular sieves, copper oxide, zinc oxide, aluminum oxide, activated alumina, and combinations of these.
U.S. Pat. No. 5,582,808 discloses providing borohydrides that are useful in reducing aldol condensation and subsequent polymer formation in caustic scrubbers. The borohydrides are believed to react with reactive carbonyls yielding more stable alcohols and a salt of the borohydride which remains water soluble, and thus is unlikely to be carried out with the hydrocarbon phase. The borohydrides of the '808 patent have the potential to reduce reactive carbonyls at a molar ratio as high as about 4:1::carbonyl:borohydride. A preferred borohydride is sodium borohydride (sodium tetrahydroborate).
It would be desirable to remove COS from process streams using an alternative process.